Electron multiplier circuits



' June 10, 1941.

G. w. WALTON 2,245,119 ELECTRON MULTIPLIER CIRCUITS Filed Dec. 5, 1938 URn-mzup Patented June 10, 1941 uurrso STATES PATENT omen V V ELECTRON CIRCUITS 1 V r 7 George Application December In Great Britain 2 Claims.

The present invention relates to electron multipliers having secondary electron emitting electrodes int-he form of grids and to circuits therefor.

An object of the invention is to provide a method of and means for injecting one or more alternating voltages into the multiplier circuits, so that these voltages will appear in the output circuit of the multiplier. In particular the invention provides a method whereby a carrier frequency can be injected into a circuit of the multiplier, so that the output of the multiplier will consist of a carrier frequency modulated in amplitude in accordance with the intensity variations of the primary electron stream of the multiplier. The method according .to the invention enables a carrier modulation of 100% to be achieved, which is of great importance when the multiplier is employed in a televisiontransmitter.

The invention is based upon the following discoveries relating to the properties of such gridelectrode electron multipliers.

An electron multiplier comprises a cathode, which may be photo-electric or thermionic, an anode or collecting electrode, and interposed between them a number of secondary electron emitting grid electrodes placed one after the other in the path of the current through the multiplier.

If the potential difference between the anode and the immediately preceding secondary emitting electrode is progressively increased from zero, the potential of the former being more positive than that of the latter, the anode current of the device is found to increase from a very low value in substantially linear manner up to a maximum value. Further increase of the potential difference will not produce any increase in the anode current. If the anode is arranged to emit substantially no secondary electrons it is found that the slope of the linear portion of the anode current curve is relatively steep. If now the primary electron emission is halved, it is found that the maximum anode current is reached at the same value of the potential difference between the electrodes, but the value of the maximum anode curre t is substantially halved, and hence the slope of the linear portion of the anode current curve is also halved. Thus the slope will vary in a substantially linear manner with respect to the primary emission from the cathode. In Fig. 1 of the accompanying drawing there is shown a series of such curves, in which the ordinates represent the anode current I of a William Walton, Kensington, London,

England 193s, Serial'No. 244,061

December 10, 1937 (Cl. 179-171) trode.

multiplier whilst the abscis-sae represent the potential difference V between the anode and the immediately preceding secondary emitting elec- Each curve corresponds to a different value of the primary electron emission of the multiplier. The upward slope of the anode current curve appears to be due to the fact that when the potential of the control electrode is equal to that of the preceding secondary emitting electrode, substantially all secondary electrons proceeding from the latter' will return to it, so that the anode current is very low. With increasing potentials a progressively increasing number of secondary electrons are collected by the anode until practically the entire secondary emission of the preceding electrode is thus collected. A potential difference between the electrodes which corresponds to a point halfway up the upward slope will be hereinafter referred to as the mean potential difference. is indicated in Fig. 1 by the potential V1, and this potential is considerably less than the normal stage voltage between two successive electrodes of the multiplier. According to the present invention, means are provided for applying a steady potential difference between the anode and a preceding secondary emitting electrode having a value substantially equal to the mean potential difference as hereinbefore defined, together with a varying voltage, whereby the said varying voltage will appear in the output circuit of the multiplier.

The varying voltage may be a high frequency alternating voltage, in which case the anode current of the multiplier will contain an alternating current component of the same frequency, the amplitude of which will depend upon the slope of the anode current curve, 1. e., upon the primary emission. If the primary emission is changed in accordance with the amplitude of signals to be amplified by the multiplier it is clear that the slope will vary in a corresponding manner, and hence the alternating anode current will be modulated in amplitude in accordance with the signals. This obviously enables a carrier frequency to be introduced and of great value when the amplification of weak signals is to be accomplished since a low loss circuit may now be utilised as the output circuit of the multiplier. Furthermore, if suitable steps are taken to neutralise the capacity between the anode and the preceding secondary electron emitting electrode, it is possible to arrange that the output current falls to zero when the primary electron emission is zero, so that modulation can be effected.

The varying voltage in this case is preferably applied between the anode and the immediately preceding secondary electron emitting electrode, and in a preferred embodiment of the invention it is applied in such a way that the potential of the latter is kept constant whilst the potential of the former is varied about a mean potential which gives the necessary mean potenial difierence be tween the two electrodes.

Such an arrangement is very useful in the case of a television transmitter. In this case the electron multiplier has a photo cathode, and a varying light beam produces a primary emission which varies in accordance with the brightness of the picture points of the picture to be transmitted. The necessary carrier current is then introduced in the manner described above.

The varying voltage may also be representative of the signals to be amplified by the multiplier, in which case the primary emission is kept constant. This enables one to use a thermionic cathode as the source of the primary emission.

The invention will now be described with reference to the accompanying drawing in which- Fig. 1 shows the curves obtained by plotting the anode current against the potential difierence between the anode and the preceding electrode and Fig. 2 shows a circuit diagram of a preferred form of the invention. 7

Referring to Fig. 2, an electron multiplier l comprises a photo electric cathode 2, a plurality of secondary electron emitting grids 36, and an anode 7. The grids 3-6 are held at progressively increasing potentials with respect to the cathode 2, these potentials being derived from the potential divider 8. A small potential difierence, equal to the mean potential difference V1 of Fig. 1 exists between the terminals 9 so that the anode l is slightly more positive than the grid 6 by this amount. A light beam of varying intensity falls upon the cathode 2 thus setting up a primary electron emission of varying intensity. A locally generated carrier frequency is applied to the terminals l of the circuit H and is injected across the electrodes 6, 1 via the lower half of the centre tapped inductance l2 of the circuit [3. A neuage between successive grids 3, 4, 5, 6 is -120 volts, the potential V1 between the terminals 9 should be in the neighbourhood of 6 volts.

I claim as my invention:

1. An electrical circuit arrangement including an electron multiplier having a plurality of sec ondary electron emitting electrodes in the form GEORGE WILLIAM WALTON. 

